A variety of hill holder systems have been proposed in the past to automatically brake a vehicle descending a hill or to prevent a vehicle from rolling backwards while ascending a hill when the vehicle is traveling below some predetermined minimum speed value.
Such can be of particular advantage to the vehicle driver in instances where the vehicle is a truck carrying a heavy load.
One example of a motor vehicle hill holding system is disclosed in U.S. Pat. No. 4,582,184, the disclosure of which is incorporated herein by reference. The device disclosed in this patent is operative to maintain the brakes in an engaged condition when the clutch pedal is depressed and the vehicle comes to a stop on an incline by means of a linkage connection between the clutch and brake pedal.
Another example of a motor vehicle is disclosed in U.S. Pat. No. 4,650,046, the disclosure of which is incorporated herein by reference. In this patent, the hill holder device includes a undirectional sprag clutch that, when actuated, permits the drive shaft to rotate in only one direction thus preventing it from rotating in an opposite direction in the event that the vehicle begins to roll backwards down an incline.
Yet another example of a motor vehicle hill holding system is disclosed in U.S. Pat. No. 4,867,291, the disclosure of which is incorporated herein by reference. Here, a sprag clutch is employed to limit transmission countershaft rotation to only one direction which again operates to prevent the vehicle from rolling in the opposite direction as previously described.
Although the above described devices may be used to advantage they all employ complicated clutch or linkage mechanisms that are expensive to install and maintain.
In contrast to the above described prior art, the present invention employs a relatively simple fluid control valve arrangement that is responsive to a control signal derived from a selected speed condition signal alone or in combination with a selected drive train condition such as a throttle position and/or clutch position so as to cause the brakes to automatically engage when the control signal is below a predetermined value.